Episode 183 Too much amniotic fluid (Polyhydramnios)

We unpack polyhydramnios with clear guidance on measurement, risk, and choices, showing why most mild cases near term are benign while outlining when to call in fetal medicine. Practical steps help parents and midwives stay calm, plan safely, and avoid unnecessary interventions.

• definitions of polyhydramnios by single deepest pool and AFI
• pros and cons of SDP versus AFI measurement
• thresholds for mild, moderate, severe excess fluid
• idiopathic cases late in pregnancy and prevalence
• screening for gestational diabetes and infection
• key risks: unstable lie, cord prolapse, postpartum haemorrhage
• induction debates, continuous monitoring, and individualised plans
• when to refer to fetal medicine and what they assess
• amnioreduction indications, risks, and diagnostic value
• reassurance for parents and guidance for midwives

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Want to know more:

https://obgyn.onlinelibrary.wiley.com/doi/10.1111/tog.70021

Thank you all for listening, My name is Florence Wilcock I am an NHS doctor working as an obstetrician, specialising in the care of both mother and baby during pregnancy and birth. If you have enjoyed my podcast please do continue to subscribe, rate, review and recommend my podcast on your podcast provider.
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Chapters

• 0:00 Welcome Back And Topic Setup
• 1:31 What Is Polyhydramnios
• 2:38 How We Measure Amniotic Fluid
• 5:47 Defining Mild, Moderate, Severe
• 8:08 Idiopathic Cases And Prevalence
• 11:12 Practical Risks In Mild Cases
• 13:26 Interventions: Monitor, Induce, Or Wait
• 15:08 When Fluid Is Moderate Or Severe
• 17:30 Infections, Tests, And Follow‑Up
• 20:05 Amnioreduction And Genetics
• 21:54 Reassurance For Parents And Midwives
• 24:28 Resources, Contact, And Support

Transcript

Speaker 1: 0:00

Hello, my name's Florence. Welcome to the Obspod. I'm an NHS obstetrician hoping to share some thoughts and experiences about my working life. Perhaps you enjoy Call the Midwife, maybe birth fascinates you, or you're simply curious about what exactly an obstetrician is. You might be pregnant and preparing for birth. Perhaps you work in maternity and want to know what makes your obstetric colleagues tick, or you want some fresh ideas and inspiration. Whichever of these is the case, and for that matter anyone else that's interested, the obspod is for you. Episode 183, too much amniotic fluid. But I'm back now. I think I just slightly run out of steam and things to talk about. But over the last few months, certain things have captured my attention, and I've thought, oh, I need to do an episode on that. So here I am, I'm back in the game.

Florence: 1:31

Today we're going to talk all about having too much, and I say that in inverted commas, amniotic fluid. You might hear this called polyhydramnius. And this was prompted by the fact that we do continuing professional development, CPD, and we have a magazine that comes out and does little updates for us. And yesterday I was reading an update about amniotic fluid. And I suddenly realised, oh, I haven't done a podcast specifically about amniotic fluid. So here we go. I'm dividing the podcast into two halves because I'm going to talk about too much amniotic fluid, polyhydramnius, today, and then too little in inverticumers, amniotic fluid or oligohydramnis in another episode. Because whilst they're both about amniotic fluid, they're actually wildly different. The first thing we need to consider when we think about too much amniotic fluid is how are we measuring this? So there are two methods of measuring amniotic fluid. One is single deepest pull, and the other is amniotic fluid index. Amniotic fluid index is where you measure fluid in roughly four quadrants of the uterus. So you put your probe on the top of the pregnant woman's uterus and you take a measurement where you see the black. So the black is the amniotic fluid. And so it shouldn't contain parts of the baby, the measurement. So for example, if a limb is in that pocket of fluid, you've got to subtract the limb and measure either side of the limb, and it also shouldn't really contain umbilical cord, so the same should measure either side of the cord. So you take four measurements, and then from that you can calculate the amniotic fluid index. The idea of this is that you're getting a more accurate measurement because if the baby's back is on one side, you're going to have very little fluid on one side of the uterus, and more fluid on the other side with the limbs. You do have to be careful when you're taking the measurements because, of course, the pregnant uterus full of fluid, and the baby is like pressing on a blown-up balloon. If you press too firmly, then you're actually pushing the fluid away from your probe and towards the other parts of the womb. So when you take the measurement, you need to press firmly enough to make good contact and get a measurement, but not so firmly that you distort the reading you're taking. An alternative way of measuring amniotic fluid is what's called the single deepest pull. And that is you scan across the womb, taking a series of measurements, and you take the largest one, the deepest one, where there is the most fluid. And again, this shouldn't contain limbs of the baby or umbilical cord, this should just be that black fluid. There is some published data, and the Cochrane review looked at this back in 2008 and suggested that it was better to use single deepest pull over amniotic fluid index. However, another trial in 2015, the SAFE trial, concluded the opposite. The amniotic fluid index actually increased rates of diagnosis of oligohydromus that is too little fluid without improving outcome. So what am I looking at when I'm given a scan report? Well, I'm usually given the single deepest pull. I'm looking to see what the number is of the single deepest pull. So having done our measurement, what do we think is excessive fluid or a bit too much fluid? Polyhydramnis, what are the definitions? So polyhydraminus is defined as an increased amniotic fluid index greater than 24 centimetres or a single deepest pull greater than 8 centimetres. So this could mean that you get a scan report that says polyhydraminus and get asked to see a doctor when you have a reading as as little as 8.2 or 8.5, and I've seen this happen. As a clinician, that's really not very helpful. I mean, what are the margins of error? And yes, we have to take a cutoff somewhere, but we need to think about how significant is this polyhydromus or excessive fluid. So there is a division designed by the Fetal Medicine Foundation to divide it into mild, moderate, and severe. So mild being between 8 to 12 centimetres, moderate being between 12 to 16 centimetres, and severe being over 16 centimeters, and that's talking about the single deepest pull. This classification is helpful to us as a clinician because it helps us decide what are we going to do about it and how concerned are we about this amount of fluid. For example, if you've got mild polyhydromnis, the most likely thing, if it's mild or moderate, that it's what we call idiopathic. We don't find an underlying cause, it's just one of those things, if you like. So I'm going to start there because really that is the majority of the polyhydramnis I see in antenatal clinic. And I think it gets very stressful for women because they perhaps polyhydramnius and come up with all sorts of scary-looking potential causes, whereas actually what we're talking about is isolated or idiopathic polyhydramnis, which is really, really common and likely to be just one of those things. Polyhydramnius complicates 1 to 2% of pregnancies. So if you have a scan at some point in your third trimester, you've got a 1 to 2% chance of the report coming out saying polyhydramnis and then being asked to see an obstetrician. So we can get back into that loop that I've previously discussed where you request a scan for a woman because you think the bump is measuring a bit big or a bit small, and the answer is that everything is fine, but you pick up polyhydramnis. Or the woman was having a scan because she's in a group that has a higher chance of growth restriction, and then you pick up polyhydramnis. So idiopathic or isolated polyhydramnis accounts for 60 to 70% of cases and 1% of pregnancies overall. So it's really quite common, and most cases are found in that third trimester. In fact, the closer you get to your due date, the more likely that if you're given a diagnosis of polyhydramnis that it is this idiopathic isolated. So the big question is if we pick up mild polyhydramnis at the end of pregnancy, what should we do? So the first thing we usually do is get a woman to test her blood sugars because one of the commonest things we need to exclude is maternal gestational diabetes. Once we've done that, if it's mild polyhydromnis, we're then left with the question: does this really matter? And in my view, if it's mild to moderate, the main thing I want to know is the presenting part, so usually the head of the baby, going down into the pelvis. If you have a bit of extra fluid, you're more likely to have a baby that is unstable lye. It doesn't settle down nicely, getting prepared for birth. And you can take a listen to episode 144, all about unstable lie. And what we worry about is the chance, higher chance of cord prolapse. That's where the umbilical cord comes down through the cervix below the head or bottom of the baby. And again, I've got an episode on cord prolapse, episode 91, so you can take a listen. But if it's mild polyhydramnis and the baby's in a good position getting prepared for birth, should we do anything? I'm talking here about a pregnancy where the baby is a normal size and we've got isolated mild to moderate polyhydramnis. When I look down at the list of associations listed in this little update document I was reading, a lot of the associated complications could be of our own making. For example, in the maternal column there's increased induction of labour, prolonged first stage of labour, increased assisted vaginal birth, postpartum hemorrhage or excessive bleeding, malpresentation, so I've just talked about that, the baby being in the wrong position, increased emergency caesarean, unsuccessful vaginal birth after cesarean. All those are kind of things that we might end up doing. And then things that are less likely to be our impact, placental abruption, preterm birth, and preterm pre-labour breaking waters. So a lot of that is us potentially feeling like we should do something and perhaps making some complications. And in terms of what to do, the article lists three options, which is do nothing, that's always the default, expedite delivery, so that's offering induction or cesarean birth, or amnioreduction, that's when you try and drain off some of the amniotic fluid. And I'll talk about that a little bit later when I'm going to talk about more severe polyhydromnis, because that really is not something that we would suggest at all for mild to moderate polyhydramnius. So that leaves us with do nothing or delivery. And this is quite difficult because some studies suggest that if you induce early, you actually increase complications and less good outcomes. And other studies suggest the opposite: that perhaps if you plan an induction, you might reduce the rate of complications in labour. So the answer is we don't really have a good answer. Mild isolated polyhydramnis doesn't link to adverse outcomes apart from the increased risk of emergency caesarean. And therefore, although we may discuss induction, I'm not sure whether that is the right answer. NICE, the National Institute of Clinical Excellence, suggests that we should recommend continuous fetal heart monitoring, say CTG, in cases of polyhydromous, regardless of how severe it is. But if you listen back to my episodes on fetal heartbeat monitoring, you'll know that that's debatable. So, in conclusion, the authors of this update I was reading suggest maybe offer induction from around 40 weeks, having discussed the pros and cons and individualise things, which I would hope we would be doing always. After birth, it could become more obvious that there is actually an underlying issue with the baby. However, this is very unlikely with isolated mild polyhydromnius. They quote a 1% chance. So if you imagine 1% of women will have mild polyhydramnis, and most of those will be idiopathic, and then 1% of those babies will have a problem, that's a one in a thousand chance that the baby will have some sort of issue picked up in the neonatal period. Now I'm going to turn to more moderate and severe polyhydramnis because this is definitely more complicated and needs more specialist support during pregnancy. For example, women with severe polyhydramnis, remember that's more than 16 centimetres single deepest pull, have around a 30% chance of the baby having some sort of anomaly. So even then, still most likely not, but there's a much higher chance. So if you have something going on with the baby's ability to swallow and not be able to recycle this fluid, then you end up with polyhydramnis. So usually if you have severe polyhydramnis, you'll be offered a scan with a specialist fetal medicine consultant who will do a full top-to-toe check of your baby to have a look and see if they can see the stomach bubble. That's normally a little black circle on the scan. What does the baby's face and jaw and chin profile look like? What do the baby's kidneys and bladder look like? And this is also polyhydromnis that's more likely to be picked up earlier in pregnancy. So, for example, if at your anomaly scan you already have excessive fluid, that would be more suggestive that there could be something wrong with the baby, and that you need that expert fetal medicine scan. The other thing that can sometimes cause excessive fluid is an infection. So when they're scanning, they'll also be looking at your baby to see if there are signs of swelling in the baby, so-called high drops, because this suggests an underlying infective possibility. So you're likely to be offered a series of blood tests, for example, checking for some specific viruses called a torch screen, checking for toxoplasmosis, rubella, that's German measles, cytomegalovirus, CMV, and herpes. And we'll also check for parvovirus, which is an infection that does typically present with high drops, so swelling in the baby and excessive fluid. We will also be testing for and trying to exclude maternal diabetes by checking blood sugars. So, what will happen is you'll have an expert fetal medicine scan, or possibly several scans, a series of blood tests to try and find the underlying reason why there is severe polyhydramnis, moderate or severe polyhydromnis, and you'll probably have some scans later in your pregnancy to follow it up. Some anomalies, such as trachea esophageal fistula, that's where the wind pipe and the swallowing pipe, the esophagus of the baby are connected, can be quite difficult to detect antenatally. And so they'll look and check that when your baby's born, if they're suspicious of that. That is something that would require surgery once the baby's born, but is a relatively straightforward correction with a good outcome. I mentioned earlier something about amnio drainage. Women with severe polyhydromnias, the womb gets bigger more quickly, can be very, very tense, almost like a drum, and very uncomfortable. And these women are at a higher chance of going into labour early as well as being very uncomfortable, and therefore, sometimes amniotic fluid drainage is considered, and that again would be done by a fetal medicine consultant. They give a little local anesthetic and they literally just pop a drain through the skin into the womb and drain off some of the fluid. We don't like to do that unless it's absolutely necessary because that in itself carries a risk of introducing infection, of course, and also preterm birth, because it could trigger going into labour. Of course, if you're taking off some amniotic fluid, you can then test it in the same way that we might do for an amniocentesis. Grow some of the fetal skin cells from that amniotic fluid and use that for genetic testing. So that can be another reason why amniotic drain. Is considered. Okay, so what's the zesty bit from all this? I think the main thing is don't be scared. If you're told you've got too much amniotic fluid and it's mild, look at your scan report, see what that level is, and if it's mild, say it's between 8 and 12, don't worry too much. Go to your obstetric appointment, understand you're going to be tested for diabetes, and that most of the time this is mild, it's not significant, it's not going to mean anything for you or your baby's outcome, it's just going to be one of those things. If it's moderate to severe or early onset in pregnancy, then you probably are going to need some specialist support in your pregnancy. But again, you're going to go and see some absolute experts and they will help and look after you during the remainder of your pregnancy. If you're a midwife, then I think this is also important context because if you're a midwife seeing a woman with mild polyhydramnis, that woman is quite likely to have looked it up and be scared about what may be wrong with her baby. So being able to confidently reassure her that this is most likely to be idiopathic, most likely not to have much bearing on the outcome of the pregnancy, then you can be a really good, confident reassurance for that woman. And if you're looking after a woman who has moderate to severe, then again you're alongside her telling her what might happen in terms of testing and fetal medicine appointments. I'm going to put a link to the article I mentioned in the show notes. It's not open access, but your midwife and doctor would be able to show you a copy because they will have access through their organisational libraries or membership of their professional organisations. I very much hope you found this episode of the Obspod interesting. If you have, it'd be fantastic if you could subscribe, rate and review on whatever platform you find your podcasts, as well as recommending the Obspod to anyone you think might find it interesting. There's also tons of episodes to explore in my back catalogue, from clinical topics, my career and journey as an obstetrician, and life in the NHS more generally. I'd like to assure women I care for that I take confidentiality very seriously and take great care not to use any patient identifiable information unless I have expressly asked the permission of the person involved on that rare occasion when it's been absolutely necessary. If you found this episode interesting and want to explore the subject a little more deeply, don't forget to take a look at the programme notes where I've attached some links. If you want to get in touch to suggest topics for future episodes, you can find me at the Obspod on Twitter and Instagram, and you can email me theObspod at gmail.com. Finally, it's very important to me to keep the Obspod free and accessible to as many people as possible. But it does cost me a very small amount to keep it going and keep it live on the internet. So if you've enjoyed my episodes and by chance you do have a tiny bit to spare, you can now contribute to keep the podcast going and keep it free via my link to buy me a coffee. Don't feel under any obligation. But if you'd like to contribute, you now can.

Speaker 1: 26:32

Thank you for listening.com.